Fallow Land Requirements

Intermediate

NS MS S VS

1.0 1.0 1.0 1.0

1.0 1.1 1.1 1.1

1.0 1.2 1.3 1.4

1.0 1.2 1.3 1.5

In their natural state, many tropical soils cannot be continuously cultivated without undergoing degradation. Such degradation is marked by a decrease in crop yields and a deterioration in soil structure, nutrient status and other physical, chemical and biological attributes.

Under traditional farming systems, this deterioration is kept in check by alternating some years of cultivation with periods of fallow. The intensity of the necessary fallow is dependent on level of inputs, soil and climate conditions and crops. However the prime reason for incorporating fallows into crop rotations is to enhance sustainability of production through maintenance of soil fertility.

4.3.1 Natural BushIGrass Fallow Land Requirements

Maintenance of nutrient fertility of land, cultivated with subsistence low inputs LUTs, is generally been achieved through natural bush or grass fallow as a means of soil fertility regeneration.

The natural bush or grass fallow requirements have been derived for the inventoried environmental conditions for four main groups of crops: cereals, legumes, roots and tubers, and banana and sugarcane (FAOIIIASA 1991). The environmental frame used consists of individual soil units, thermal zones, represented by warm (Tmean > 200C), moderately cool (Tmean 15-200C) and cool temperature regimes (Tmean 5- 150C), and moisture regime, represented by length of growing period zones < 90, 90-120, 120-180, 180-270 and > 270 days.

Part A of Table 4.4 presents reference fallow requirements (F) as function of soil fertility.

This factor is expressed as percentage of time during the fallow-cropping cycle (i.e.

tf/(tc+tf)x100) the land must be put under fallow. The reference values have been used as basis for the scoring of natural bushlgrass fallow requirements for a particular soil unit and crop group at low level inputs. For Fluvisols and Gleysols fallow factors are lower because of their special moisture and fertility conditions.

At the intermediate level of inputs the fallow requirements are taken as one third of those at the low level. At the high level of inputs, fallow requirements are set at 10%.

4.3.2 Forage Legume Fallows

Forage legumes provide a faster means for soil regeneration compared to natural bushlgrass fallows (Hague and Jutzi, 1984; Reeves and Ewing, 1993; Mohamed-Saleem and Fischer, 1993). Sown forage legume fallows can provide a head start to the time required for natural bushlgrass fallows to establish. This offers better early protection to the soil from torrential rains at the start of the rainy season. Also the forage legumes provide nitrogen accrual in the soil through biological fixation which in turn is enhancing the quality of livestock feed.

Forage legumes in a crop/livestock production system can be used in different ways. The following assumptions have been made to enable assessment of forage legumes in fallows:

(i) Unintempted forage growth from the time of sowing until strategically grazed in situ.

(ii) Forage legume fallow benefits are similar for those of natural bushlgrass fallows with the exception in the case of forage legumes fallow where there is extra impact on soil fertility through increased organic matter and nitrogen resulting from additional biomass production and biological nitrogen fixation.

(iii) Capture of the accrued soil nitrogen by crops from the forage legumes is the main objective of forage legume fallow and other benefits are adjunct to nitrogen build up.

Soil nitrogen accrual potentials for the different legumes under West African conditions and available amount of legume-nitrogen in soil, available to crops after two years of forage

legume fallow, estimated through bioassay are summarized in Table 4.3.

Table 4.3 NITROGEN ACCRUALS FOR FORAGE LEGUMES

Based on nitrogen fixing and transferring abilities, chamaecrista, lablab and forage vigna (group I) could benefit crops up to an average of 58 kghdyear and centrosema, siratro and stylo (group 11) under similar management situation benefit crops up to an average of 68 kghdyear.

Forage legume density in the fallow and biological nitrogen accruals vary depending on the soil fertility status. Inherently fertile soils with higher nitrogen levels suppress biological nitrogen fixation and nitrophilous grasses out compete forage legume establishment. Forage legumes also poorly establish in very infertile soils. Forage legume dominance is attained in moderately fertile and infertile to moderately infertile soils.

Legume Group fallow or natural bushlgrass fallow rotations will therefore depend on the relative gain in soil nitrogen during the respective fallows and is estimated as:

Legume Type

a = Soil nitrogen deficit during cultivation b = Soil nitrogen gain during legume fallow c = Soil nitrogen gain during natural fallow

F(natura1 bush/grass fallow) has been taken from FAOmASA (1991)

Part B and C of Table 4.4 present respectively reference fallow requirements as a function of soil fertility for Group I forage legumes (chamaecrista, lablab and forage vigna) and for Group I1 forage legumes (verano stylo, centrosema and siratro). Similar to the values presented in Part A of Table 4.4, these reference values are used for scoring of fallow requirements for a particular soil unit at low level inputs.

Available N to

Table 4.4 REFERENCE FALLOW REQUIREMENTS AS FUNCTION OF SOIL FERTILITY A. Natural Bush/Grass Fallow Requirements

B. Forage Legumes Fallow Requirements (Group I: chameacrista, lablab, forage vigna)

Soil

C. Forage Legumes Fallows Requirements (Group 11: verano stylo, centrosema, siratro)

Moderately Cool

I1 Moderately fertile soils

I11 Infertile to moderately fertile soils IV Infertile soils

This section describes the soil erosion and productivity submodel, which quantifies implications of alternative land uses in terms of topsoil loss due to erosion and its impact on the productivity of land under different assumed soil conservation measures. The submodel is adapted from an existing model (FAOAIASA 1991).

Warm